NRL’s Paradigm-Changing Instrument Takes the World by iSTORM

WASHINGTON  –   U.S. Naval Research Laboratory’s (NRL) instrument, the in-Situ Thunderstorm Observer for Radiation Mechanisms (iSTORM), provides paradigm-changing observations of ionizing radiation from tropical thunderstorms aboard a NASA ER-2 high-altitude research aircraft, recently published in Nature.
 
Thunderstorms are a destructive force that cause damages costing tens of billions of dollars annually. Measuring gamma-ray emissions from thunderstorms provides an opportunity to remotely sense the electric field conditions within the cloud. Better understanding of lighting initiation can improve modeling of these thunderstorm systems.
 
These observations were performed by an international team during the Airborne Lightning Observatory for the Fly’s Eye Global Lightning Monitor Simulator and Terrestrial Gamma-ray Flashes (ALOFT) campaign in July 2023.  The campaign was led by the University of Bergen (UiB), Norway.  The NRL Space Science Division (SSD) provided one of the instruments, iSTORM, to make detailed measurements of gamma rays from above these storms.
 
“ALOFT is truly a multi-disciplinary international team that brings together many different instruments to study thunderstorms and lightning processes, which make this campaign unique to diagnose these high-energy phenomena,” said Daniel Shy, Ph.D., a recent Karles Fellow and instrument scientist.
 
iSTORM is a gamma-ray spectrometer optimized to make sensitive measurements of bright, fast transients in the nuclear gamma-ray band ranging from ~300 keV to >5 MeV.
 
NRL-designed and built, iSTORM is a highly-segmented array of fast, high-resolution inorganic scintillators. Scintillation detectors convert absorbed ionizing radiation into detectable light photons, usually in the visible light range. Silicon photomultipliers convert the scintillation light to electrical signals that are digitized and stored on board iSTORM.
 
“Its segmented array of high-resolution scintillators allows iSTORM to make sensitive and precise measurements of the energies and intensity of gamma rays emitted by thunderclouds,” Shy said.
 
NRL team members then use these signals to measure the radiation environment to estimate the intensity and energy of radiation sources.
 
“iSTORM is one of two gamma-ray instruments deployed on ALOFT, with the other instrument developed by UiB,” said J. Eric Grove, Ph.D., Principal Investigator for iSTORM and Head of the High Energy Space Environment Branch in SSD. “The two instruments are independent and complementary. When coupled they provide high-confidence measurements of what we expected would be very intense and complex bursts of radiation from these storms.”
 
“iSTORM was built to hold the detectors at ambient pressure while withstanding the ER-2 aircraft’s shock, vibration, and temperature environments,” said Alena Thompson, NRL’s lead iSTORM mechanical engineer.
 
Before the 2023 ALOFT campaign, scientists understood that thunderclouds sometimes glowed dimly in gamma rays and, on occasion, emitted very brief, extremely bright bursts of gamma rays, known as Terrestrial Gamma-ray Flashes (TGFs).
 
“Two of the most interesting results from ALOFT have changed collective views of thunderstorm electrification by showing additional underlying phenomena occurring in active storms,” Grove said.
 
Previously, glows had been observed for a few tens of seconds spanning several kilometers.  “Instead, the campaign team observed that tropical thunderstorms glow with gamma rays for hours over hundreds of square kilometers, and that the glows are complex in time and space, bubbling like a boiling pot,” Shy said. “The rapid changes in gamma-ray flux within the long-duration glow indicate that thunderstorm electric fields vary in unexpected ways.”
 
Another surprise finding recorded a new transient phenomenon that is marked by multiple, bright pulses of gamma rays with a total duration of up to a quarter of a second. The phenomenon, named a Flickering Gamma-ray Flash (FGF), appears to have intermediate properties between gamma-ray glows and TGFs and does not fit into either category.
 
“They seem to be something intermediate between gamma-ray glows and TGFs,” Grove said. “Like glows, they have no associated radio or optical signatures – which suggests they’re not closely related to lightning discharges – yet they show up as a series of bright pulses of gamma rays.  On the other hand, the extremely bright, fast pulses, the TGFs, have well-documented radio and optical counterparts and are connected to lightning.”
 
This campaign also offered an opportunity to validate measurements from space-based lightning instruments like the NOAA’s Geostationary Lightning Mapper and NASA’s Lightning Imaging Sensor on the International Space Station.
 

About the U.S. Naval Research Laboratory
 
NRL is a scientific and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seafloor to space and in the information domain. NRL is located in Washington, D.C. with major field sites in Stennis Space Center, Mississippi; Key West, Florida; Monterey, California, and employs approximately 3,000 civilian scientists, engineers and support personnel.
 
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